European Journal of Mechanics B-fluids最新文献

We compared the accuracy of volume of fluid (VOF) methods in unstructured solvers using the following five different methods: 1 - the algebraically compressive VOF method, 2 – simple coupled VOF method with Level Set (S-CLSVOF) method, 3 - interface-compressing VOF method incorporated with Laplacian filter (VOFL), 4 - isoAdvector method, and 5 - isoAdvector method incorporated with Laplacian filter (isoAdvectorL) by incorporating them into OpenFOAM®, an open-source software. To evaluate these methods under proper conditions, we compared the calculation accuracy using the optimized parameters, which are explored by Bayesian optimization. The test cases for advection accuracy of volume fraction and for imbalance of surface tension force in static multiphase fluid fields were considered. In this study, we found that the compression parameters and maximum Courant number should be adjusted to obtain high accuracy simulation according to the simulation condition in VOF and S-CLSVOF method. In VOFL and isoAdvectorL methods, the spurious current can be extremely reduced, which means that these methods are suitable for slow flow with higher Laplace number conditions.

In fluid dynamics, the Atwood number is a dimensionless parameter that quantifies the density difference between two fluids. It is calculated as $At=({\rho }_1-{\rho }_2)/({\rho }_1+{\rho }_2)$, where ${\rho }_1$ and ${\rho }_2$ represent the densities of the respective fluids. This research employs high-fidelity numerical simulations to examine the Atwood number impacts on Richtmyer–Meshkov (RM) flows triggered by a shocked forward-pentagonal bubble. Five distinct gases — ${\text{SF}}_6$, Kr, Ar, Ne, and He — are considered within the forward-pentagonal bubble, encompassed by ${\text{N}}_2$ gas. In these simulations, a third-order discontinuous Galerkin approach is applied to solve a two-dimensional set of compressible Navier–Stokes-Fourier (NSF) equations for two-component gas flows. To discretize space, hierarchical modal basis functions based on orthogonal-scaled Legendre polynomials are employed. This approach simplifies the NSF equations into a set of ordinary differential equations over time, which are solved using an explicit third-order SSP Runge–Kutta algorithm. The numerical results highlight the notable impact of the Atwood number on the evolution of RM flows in the shocked forward-pentagonal bubble, a phenomenon not previously reported in the literature. The Atwood number exerts a significant influence on the flow patterns, leading to intricate wave formations, shock focusing, jet generation, and interface distortion. Moreover, a comprehensive analysis of the these impact elucidates the mechanisms driving vorticity formation during the interaction process. Additionally, the study conducts a thorough quantitative examination of the Atwood number impacts on the flow fields based on integral quantities and interface features.

Diffraction is a fundamental phenomenon that occurs when blast or shock waves pass over sudden discontinuous surfaces. It generates a complex flow field consisting of diffracted waves, expansion waves, slipstream, contact surface, and an unstable shear layer, in addition to emitting acoustic waves. In this study, we investigated the diffraction of a blast wave passing over a series of grooved structures with different aspect ratios and geometrical shapes (rectangular, circular, and triangular) using high-speed shadowgraph images. The blast wave Mach number considered in our investigation is 1.34. The grooves feature leading-edge geometrical variations such as rectangular, circular arc, and wedge shapes positioned at various lateral locations from the exit of the shock tube. The aspect ratios of the rectangular grooves vary from 0.33, 0.5, and 0.67. The circular and triangular grooves have an aspect ratio of 0.33. The trajectories and velocities of the primary vortex are calculated by tracking the location of the vortex in the shadowgraph images. Our observations revealed that a large portion of the incident blast wave is abducted inside the groove as the aspect ratio increases in rectangular grooves, resulting in better attenuation of the blast wave. The grooves, which have circular shapes, produced weaker diffraction, which resulted in delayed and weak primary vortex. The triangular grooves produced the strongest primary vortex and have the highest attenuation characteristics among other grooves. The strength and trajectory of the primary vortex formed over the grooves strongly depend on the aspect ratio and the curvature of the leading edge for a given Mach number. Vortices generated from rectangular and triangular grooves exhibit considerable strength and longevity.

An airlift pump is a vertical tube that utilizes the buoyant effects of a gas to lift a liquid. Unlike a standard mechanical pump, the liquid flow rate through the airlift pump is not directly controlled; rather, it depends on the supplied gas flow rate, the tube length and diameter, and the relative height of the liquid supply free surface (submergence ratio). The present study uses the commercial CFD code ANSYS CFX to model the isothermal, 3D, transient flow in an airlift pump using water and air. The model applies pressure boundary conditions at both ends of the tube and specifies the mass flow rate of air through multiple openings in the side of the tube. The bottom of the tube is an inlet of water only and the outlet is a two-phase flow opening. A time-dependent, homogeneous, VOF two-phase RANS CFD modelling approach is used with the air treated as an ideal gas. This work found that a complete 3D domain was necessary for consistent prediction of the airlift performance and physically realistic two-phase flow structures. Statistical analysis of the two-phase flow structures was applied to characterize airlift pump instability and better understand the physics of the airlift pump.

Transport of particles in flows is often modeled in a combined Eulerian–Lagrangian framework. The flow is evaluated on an Eulerian grid, while particles are modeled as Lagrangian points whose positions and velocities are evolved in time, resulting in particle trajectories embedded in the time-dependent flow field. The method essentially resolves the flow field in complex geometries in detail but uses a closure model for the particle dynamics aimed at including the essential particle–fluid interactions at the cost of detailed small-scale physics. Rarefaction effects are usually included through the phenomenological Cunningham correction on the drag force experienced by the particles. In this Lagrangian point-particle approach, any explicit reference to the finite size and the shape of the particles, and their local orientation in the flow field, is typically ignored. In this work we aim to address this gap by deriving, from fully-resolved Direct Simulation Monte Carlo (DSMC) studies, heuristic or closure models for the drag force acting on prolate and oblate spheroidal particles with different aspect ratios, and a fixed orientation, in uniform ambient rarefied flows covering the transition regime between the continuum and free-molecular limits. These closure models predict the drag in the transition regime for all considered parameter settings (validated with DSMC data). The continuum limit is enforced a priori and we retrieve the free-molecular limit with reasonable accuracy (based on comparisons with literature data). We also include in the models the capability to predict effects related to basic gas-surface interactions via the tangential momentum accommodation coefficient. We furthermore assess the validity of the proposed closure model for particle dynamics in proximity to solid walls. This investigation extends our previous work, which focused on small aspect ratio spheroids with exclusively diffusive gas-surface interactions [see Livi et al. (2022)]. The derived models are obtained for isothermal, subsonic flows relevant for particle contamination control in semiconductor manufacturing.

Flow control techniques used on wind turbines have been shown to significantly increase energy generation when compared to traditional wind turbines. Although various flow control methods have been introduced in the last two decades, the comparison between these methods is still the least conducted by researchers. Therefore, the present study aims to evaluate the performance of an airfoil utilizing both single and dual passive flow control methods, such as droop, flap, microcylinder, slot, and spoiler with optimal parameters. In this study, a numerical model was developed and applied with the same boundary conditions as those in the experiment. The results of the developed numerical simulation were then validated with experimental and other numerical studies. Mosaic mesh was utilized and the results were compared with conventional mesh types. Even though the mosaic mesh requires a lower number of computational elements, it demonstrated higher computational accuracy when compared to hexcore, polyhedra and tetrahedral type meshes. After obtaining an accurate numerical model, parametric studies were then conducted. The findings mainly highlighted that the airfoil with a microcylinder consistently generated higher performance than droop, flap, spoiler, slot and conventional airfoil. The mean relative improvement was about 2.6%. In an extensive study, eight combinations of flow controls were proposed and evaluated. The highest performances were achieved with the combination of microcylinder and flap, up to 27.9% and the combination of microcylinder and slot, reaching up to 50.2%, for low and high AOAs, respectively.

This review delves into the dynamics of fibre-laden turbulent flows, a field that has garnered substantial attention due to its relevance in both natural and engineering contexts. The focus here is mainly on finite-size fibres, those exceeding the Kolmogorov scale, diverging from the commonly studied smaller ones. The study synthesises current understanding of the behaviour and organisation of both rigid and flexible finite-size fibres within turbulent flows, underscoring the added complexity these anisotropic particles introduce compared to their spherical counterparts. The influence of the length, the curvature and the inertia on the dynamics of rigid and flexible fibres is addressed. Fibre-based novel experimental methods, such as Fibre Tracking Velocimetry, are highlighted. Ultimately, this paper seeks to provide a clearer picture of the intricate dynamics at play in fibre-laden turbulent flows and their practical implications in various fields.

This paper explores the combined influence of an axial electric field and a perpendicular magnetic field imposed on rotating micro-parallel plates immersed in an electrolyte solution. A specialized computer program was developed to solve the velocity as well as the EDL potential fields using the finite difference method, employing the Debye-Hückel (DH) approximation to linearization the EDL potential. The study examines the influence of various non-dimensional parameters, including rotational speed ($\omega$), Hartmann number (Ha), Debye-Hückel parameter ($\kappa$), and the non-dimensional parameter ‘S’, on axial, and transverse velocities, wall shear stress, and net flow rate. Results demonstrate that, both velocity components decrease with increased rotational speed and Hartmann number, while the net flow rate increases with the Debye-Hückel parameter for both rotating and non-rotating systems. The impact of these parameters on shear stress was also analyzed. Analysis of Ekmann spirals in the velocity plane revealed closed spirals at a higher rotational speed and open spirals at lower speeds, with spiral size reducing as rotational speed increases.